I am sure to many an ear this will sound flaky but I recently heard of at least two different people who, during their meditations (non-Buddhist) , were accosted by malevolent entities. Although I never thought of it until I heard of these people's experiences I was wondering if the "Namo tassa"s, the vandanas and the refuges I customarily recite before my meditations have any protective effect. The idea that this may be so is not unheard of in the tradition (take for example the Paritta chants and suttas like the Ratana sutta) but I was wondering if anyone had heard of anything concrete. Sukhitaa hontu!

To avoid all evil, to cultivate good, and to cleanse one's mind — this is the teaching of the Buddhas.-Dhp. 183

Then the first yakkha, ignoring the second yakkha, gave Ven. Sariputta a blow on the head. And with that blow he might have knocked over an elephant seven or eight cubits tall, or split a rocky crag. But right there the yakkha — yelling, "I'm burning!" — fell into the Great Hell.

Now, Ven. Moggallana — with his divine eye, pure and surpassing the human — saw the yakkha give Ven. Sariputta a blow on the head. Seeing this, he went to Ven. Sariputta and, on arrival, said to him, "I hope you are well, friend Sariputta. I hope you are comfortable. I hope you are feeling no pain."

"I am well, friend Moggallana. I am comfortable. But I do have a slight headache."

"How amazing, friend Sariputta! How awesome! How great your power and might! Just now a yakkha gave you a blow on the head. So great was that blow that he might have knocked over an elephant seven or eight cubits tall, or split a rocky crag. But all you say is this: 'I am well, friend Moggallana. I am comfortable. But I do have a slight headache'!"

"How amazing, friend Moggallana! How awesome! How great your power and might! Where you saw a yakkha just now, I didn't even see a dust devil!"

“I am a man of virtue. I have come here with the purest intentions – to practice Dhamma for my own spiritual benefit, and the benefit of others. Despite that, you threaten to pound me into the ground, giving no thought to the consequences of such an evil deed. Don’t you realize that it will drag you into hell where you will reap the terrible misery you have sown? Rather than feel concerned for myself, I feel very sorry for you – you’ve become so obsessed with your own authority that it’s now burning you alive. Can your potent powers withstand the effect of the grave act you are about to commit? You say you exercise sovereign authority over this mountain, but can your magic powers override Dhamma and the laws of kamma? If your powers really are superior to Dhamma, then go ahead – pound me to death! I’m not afraid to die. Even if I don’t die today, my death remains inevitable. For the world is a place where all who are born must die – even you, blinded as you are by your own self-importance. You are not above death, or the laws of kamma that govern all living beings.” The mysterious being stood listening, rigid as a statue, the deadly metal club resting on his shoulder as Ãcariya Mun admonished him by means of samãdhi meditation.

For me, next to the suttas - along with the commentaries, this is about as concrete as it gets.